Use Physical Standby for testing

Assuming a guaranteed restore point was used, after the Standby has been Primary for a while, this will return it to being a Standby

shutdown abort;
startup mount;
select name from v$restore_point;
flashback database to restore point DRTEST;
alter database convert to physical standby;
shutdown abort;
startup nomount;
alter database mount standby database;
alter database recover managed standby database disconnect;